1. Technical Field
The present invention relates to a device for monitoring the integrity of a diaphragm in a diaphragm transport unit made of one or a plurality of layers, having a resistance device connected to one layer and having a measuring device interconnected to a current source via a link.
2. Background Information
In the chemical and pharmaceutical industries diaphragm pumps are in widespread use for delivering and metering liquids. The pump action is achieved by a periodic deflection of a diaphragm. In the following, by the word diaphragm is to be understood a flexible layer which, by itself or in connection with other layers, separates a transport chamber from a drive chamber in a diaphragm transport unit. The deflection of the diaphragm can be effected mechanically, e.g. by a moved metal flange, pneumatically or by hydraulic fluid. Thus one surface of the diaphragm is in direct contact with the transport medium. Depending on the area of application, the transport medium may be poisonous, toxic and also inflammable. For safety reasons and so as largely to rule out injury to people and damage to the environment and plant components, one endeavors to prevent the escape of transport medium by regular maintenance of the transport device and by monitoring the process. The diaphragm itself is subject to great alternating stress, and is mostly the component having the greatest rate of failure in these installations. The alternating stress is predefined by operating parameters such as pumping capacity, chemical composition and the temperature of the transport medium. In order to be sure to avoid the escape of critical liquids, the pump diaphragm is replaced after a certain operating time, within the scope of maintenance work. However, in doing so, one often replaces an intact diaphragm which still has a relatively high remaining service life. For reasons of safety, and so as to be able to conduct this maintenance work as efficiently as possible, an attempt is made to detect ahead of time impending damage by measurement technology, using suitable measuring devices. One should be able to ascertain a diaphragm rupture as far ahead of time as possible. The diaphragm should be changed safely before its failure, but with only a short service life remaining. In any case, an incipient rupture of the diaphragm, or a leak which has appeared, have to be detectable by the process control system.
Various devices are known for making damage in an agitated diaphragm technically measurable.
In German Laid-Open Document 37 29 726 a diaphragm having a plurality of layers is proposed, in which a membrane gap is furnished with a detector and monitors the gap. If one of the layers is destroyed, the fluid penetrates the gap and the leak detector recognizes what the trouble is. However, the detector records the leak only when enough liquid has leaked out. Only when the two layers of the diaphragm have been pushed sufficiently far apart, and liquid has reached the detector, is the damage recognized.
In German DE 40 27 027 C2, for the purpose of recognizing a location of rupture in a diaphragm made of a plurality of layers, monitoring the space between two layers is also proposed. The pump is driven hydraulically. The space between the two layers is set up in such a way that underpressure prevails compared to the atmosphere. The space is originally free of liquid. When one of the outer diaphragms is damaged, either hydraulic fluid or transport fluid penetrate into the space between the diaphragm layers. As a result, the pressure in this space changes. (It changes) either toward the discharge pressure or toward the working pressure. The pressure change is measured by a measuring transducer in a measuring chamber connected to this space. A pressure change always occurs when one of the two layers of the diaphragm pair ruptures. However, it can happen that the connecting duct to the measuring chamber does not open, and a status signal is released too late or not at all. Because of that, damage cannot be reliably prevented, because the remaining layer of the diaphragm pair, can, for a limited duration to be sure, still safeguard the pumping function, but it should not be excluded that it might come to a leak in the second diaphragm in rapd succession. In a hydraulically driven pump hydraulic fluid can get into the transport chamber. This is always a disadvantage if high purity of the delivered fluid is important.
In WO 95/06205, for predicting a fault of a pump diaphragm, using an electrically conductive fiber made of polytetrafluoroethylene is proposed. This conductive fiber is embedded in a diaphragm layer of the same material. The fiber is dimensioned in such a way that it essentially covers the entire area of the diaphragm in the form of a spiral or double spiral, or in zigzag shape. The ends of the fiber are brought out at the edge, and can be connected to an electrical measuring device. The resistance of the fiber in ohms is measured. As soon as the diaphragm layer shows signs of fatigue or rupture, these are transferred to the fiber. Since the fiber is made of the same or very similar material as the diaphragm itself, the sagging life of the diaphragm is just about the same as that of the fiber. As a result of the signs of fatigue or the fissures of the diaphragm, the conductivity of the fiber changes, and thereby a fissure or beginning rupture can be detected with measuring technology and can be signaled. A disadvantage is the very expensive production of the conductive plastic fiber and its embedding into the diaphragm. A further disadvantage is the dependence of the quantity to be measured on the temperature of the transport medium. The post-connected electronic signal enhancer has to distinguish between a change of resistance, caused by a sign of fatigue, an extension on account of pump lift and a temperature-dependent change of resistance.
The object of the present invention is based on creating a device with the aid of which the integrity of a diaphragm in a diaphragm transport unit can reliably be monitored in a simple manner. The device should be refined in such a way that the monitoring is as independent as possible of the temperature and flow of the transport fluid in the transport chamber, and independent of a long-term deviation of the measuring device itself.
The object is attained by a device in which at least one layer of a multi-layer diaphragm is interconnected to a resistance device and to a measuring device having a current source, according to the present invention, by the measuring device""s working in the manner of a Wheatstone bridge in which the resistance device forms a bridge resistance in at least one bridge arm and an incipient rupture of a layer of the diaphragm effects a change in the galvanometer arm voltage.
On account of the change in the electrical resistance of a resistive sensor, connected to a diaphragm layer, which is positioned in at least one bridge arm of a resistance measuring bridge, the integrity of a diaphragm in a transport unit can be monitored in a technically simple manner. The sensitivity of the measuring bridge makes it possible reliably to detect an incipient rupture, spreading in one of the layers of the diaphragm, already in the preliminary stages. Wheatstone bridges are precision measuring devices for measuring resistances by comparison with built-in resistances of known values. In the present case, the bridge resistances of the measuring device can be advantageously integrated in a technically simple way with the diaphragm transport unit. This circuit arrangement of the kind of the Wheatstone bridge leads to a simple and space saving construction of the monitoring unit. In an advantageous further refinement, the bridge circuit makes possible the positioning of the bridge resistances in such a way that the disturbing influence of the temperature of the transport fluid and also the long-term drift of the resistive sensor can be eliminated.
The resistance device can be formed from various suitable electrically conductive structures, such as a metal structure, a conductive nonwoven fabric structure, a conductive adhesive structure or a structure made of conductive plastic. Production of these conductive structures can take place, for example, by etching, cutting, stamping or printing. The conducting structure can be fixed adjacently to one surface of a layer diaphragm, and it can also be embedded in the layer.
Advantageously, the resistance device is a surface conductor loop whose material is made of a conductive plastic. Such materials can be easily printed flat on a diaphragm layer using silk-screening. Commercial silver conductive adhesive on a polyester substrate is suitable, for example. Especially preferred is a surface conductor loop made of an electrically conductive elastomer. This can also be fastened to the diaphragm layer in a technically simple manner, for instance, by silk-screening or prevulcanizing. By electrically conductive elastomer is meant an elastomer formed by cross-linkage of a plastically deformable rubber with the addition of conductive fillers. The specific resistance of this elastomer conductor loop can be set by the concentration of the conductive filler. For the resistance device, an elastomer conductor loop has proved effect, which is formed from a material having a specific electrical resistance of less than 300 ohm-cm.
It is of advantage if the resistance device is formed from two electrical resistances of a surface conductor loop, and these are adjacently connected to one layer of the diaphragm. The Wheatstone bridge is then formed from these and two electrical reference resistances not connected to the diaphragm. In this arrangement, the influence of the temperature of the transport medium has an effect in both bridge arms on both resistances S1 and S2 at the same time. On the assumption that S1 and S2 demonstrate the same temperature properties, the galvanometer arm voltage between points B and D of the Wheatstone bridge is independent of the temperature of the transport medium. The long-term properties of electrical resistances S1 and S2 also act symmetrically. This is meaningful since the electrical resistance of an elastomer conductor loop decreases when a mechanical stress is put on the elastomer conductor loop. The electrical resistance of these materials is also timewise unstable and changes with aging. On account of the preferred symmetrical positioning in two bridge arms, this long-term deviation, also denoted as drift, is eliminated. On the other hand, an incipient rupture of the diaphragm, which changes the value of resistances S1 or S2, is reliably recognized. The advantage of the Wheatstone bridge is that even a very small amount of damage of the printed conductor cross-section of the surface conductor brings about a significant change in the galvanometer arm voltage. Thus, an incipient rupture can be detected very early.
It is also of advantage if the resistance device is formed from four electrical resistances of a surface conductor loop, which is adjacently connected to one layer of the diaphragm, and the Wheatstone bridge is formed from these resistances. In this way, the resistance device is distributed symmetrically to four sectors of the diaphragm. In addition to the above-named advantages, asymmetrical flow conditions in the transport chamber, and a disturbing influence of the measuring signal that goes along with it on account of the temperature of the transport medium, can now also be offset.
It is of particular advantage if a layer of the diaphragm is formed as a protective layer lying toward a transport chamber, and the surface conductor loop is prevulcanized or bonded to an inner surface of the protective layer averted from this transport chamber. The development of the conductor loop as a surface conductor and the positioning of this surface conductor at the protective layer has the effect that, during the operation of the pump, the diaphragm and the sensor are submitted to approximately the same bending alternating stress. Since the failure of a diaphragm is almost always preceded by a rupture spreading from the transport chamber in the direction of the diaphragm, damage can be recognized early. A rupture in the protective layer changes the conductivity of the surface conductor and is detected with measuring technology. Leakage of transport liquid from the pump is prevented.
It is quite especially advantageous if the protective layer of the diaphragm is made of polytetrafluoroethylene and is connected to the rubber-elastic diaphragm disc. This construction of the diaphragm makes possible, on the one hand, very good protection from aggressive chemical media, and on the other hand it makes possible the elasticity of the elastomer for a long operating time. Almost always the protective layer of the diaphragm fails before the rubber-elastic diaphragm disc breaks. The monitoring conductor positioned at the inner surface of the protective layer thus detects an incipient rupture before the entire diaphragm fails.
In an especially preferred way, the surface conductor loop is made of a material composition which has the same elastomer as the diaphragm disc and contains an electrically conductive filler. Thus the connection of the layers to the sensors is produced very well. Production of the diaphragm is thereby made simpler. The preformed sensor element is simply placed between the layers of the diaphragm during molding as a non-vulcanized or fully cured part. By adhesion or vulcanization a firm adjacent connection is created between sensor and diaphragm layer.
Here, electrically conductive fillers such as graphite particles or metal particles are used, and, especially preferred, conductive carbon black.
It can be of especially great advantage if the material of the surface conductor loop has a flexural strength which is less than the flexural strength of the material of one of the layers of the diaphragm. Whilst the material properties of the surface conductor are chosen in that way, a rupture occurs in the sensor before a fissure in the diaphragm layer begins to develop. In other words, the monitoring device signals an expected material fatigue of the diaphragm before an aspect of the damage (becomes apparent). In particular, in application areas having critical safety considerations, the diaphragm can in this way be changed with great safety before its failure, yet without an excessively high remaining service life.
In another advantageous development the surface conductor loop is placed, in a meandering shape, into a ring-shaped deformation area of the diaphragm. By doing this, it is possible that an incipient fissure of the diaphragm is recognized very early. As seen in the cross-section of the diaphragm, a fissure most often begins on the surface of the protective layer facing the transport fluid. From there, it broadens out into deeper lying areas of the layer. From an upper view of the diaphragm the longitudinal growth of the fissure takes place mostly in the circumferential direction. Depending on the distance between the meanders of the surface conductor loop, a still very small damage to the diaphragm can be reliably recognized. The shape of the meander, however, also makes it possible that the radial spreading of a fissure is reliably detected.
The measuring device can advantageously be integrated into the diaphragm transport unit, that is, the circuitry of the measuring device is accommodated directly in a subsection of the housing of the diaphragm transport unit. In case of damage, only the diaphragm together with the resistance device are changed. The measuring device may contain signal preprocessing and may have available a standardized electrical interface. In this manner, a plurality of transport units can be connected to one electronic bus system. A process control system can therefore monitor a plurality of diaphragm pumps simultaneously. For monitoring the diaphragm, the measuring electronics can contain a microcomputer or microcontroller. This digital signal processing is of advantage, for example at start-up of the device. The bridge balance of the measuring device can simply take place in situ by starting an initialization program which sets the diagonal voltage of the measuring bridge to zero. The same applies to changing the diaphragm in connection with maintenance work.
It is also preferred that a current monitoring device monitor the time average of the current in the link. This makes possible monitoring the performance reliability of the monitoring device itself. For example, a conductor break in the measuring unit leads to a change in the mean current in the link between the measuring device and the current source. In connection with a process control system, this makes remote monitoring of the monitoring unit possible. Even in the rare case when both surface conductor loops are damaged simultaneously and in equal measure during a fissure, this current monitoring device makes it possible that diaphragm damage is reliably recognized in this case too.
One can obtain a very sensitive monitoring device by positioning the resistance device in the form of a half-bridge. In this case it is favorable if the resistance value of the reference resistances (R1, R2) is at least ten times as great as the resistance value of the surface conductor loop (S1, S2).
It is advantageous if the surface conductor loop is laid out in such a way that the ends of the loop are brought to the outer edge and are provided with electrical connectors, e.g. a plug connector. Thereby the sensor diaphragm can be simply connected to a measuring device, for instance a resistance measuring bridge. Naturally, it is also possible to design the connector as a contacting element, so that the electrical connection of the resistance device can take place directly by contacts at the housing halves in the clamping area of the diaphragm.
Quite especially preferred is an elastomeric conductor loop having similar mechanical properties to the rubber-elastic diaphragm itself. This is obtained by adding conductive carbon black to the elastomer as electrically conductive filler. That permits the production of an elastomeric conductor loop in which the tensile strength lies between 8-25 N/mm2, the elongation at break lies between 50 and 400% and the specific resistance is actually lower than 300 ohm-cm. Compared to other conductive plastics, as used, for instance, for electrically conducting floors, the electrical resistance of the elastomeric conductor loop is low. With regard to the measured value processing, the electrical resistance of the conductor loop lies in a favorable range.
The present invention is explained in detail below, using exemplary embodiments represented in the drawings.
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